We ended up having the video edited to add a play button to the first frame.

It's not possible to be notified of when the user enters/exits VR mode.

Here's why: The Android Google Cardboard VR SDK is built to be accessible for easy integration in content-viewing apps. It is not built to be used in a custom-VR video viewing application. Here's a thread on Github about the topic

Due to the limitations above, we very quickly rewrote the application in Unity (still using Cardboard SDK)

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